1. Field of the Invention
The present invention relates to a water drop detection sensor for detecting the presence or adhesion of water drops upon a light-permeable substrate such as a window glass used in an automobile or other vehicle. In addition, the water drop detection sensor can further detect the ambient brightness in the surroundings as well as the sound of water drops. Further, the sensor can transfer a water drop detection signal and a processed signal thereof between equipment installed within the vehicle.
2. Description of Related Art
Conventionally, attempts have been made to detect water drops present upon or adhering to the front glass (windshield) of a car so as to automatically operate or initiate a wiper, and an example of a substrate having such a detection function is known, having the structure as shown in FIG. 19 attached.
In FIG. 19, a pair of prisms 201 and 202 are attached onto the surface of a transparent substrate 200--such as a glass plate or the like--into which light from a light source 203 is introduced through the prism 201 at an incident angle such that the incident light undergoes total internal reflection. By setting the incident angle so that total internal reflection will not occur if a liquid such as water is present upon or adheres to the surface of the transparent substrate 21)0, the amount of light that undergoes the total internal reflection changes depends upon the existence of the liquid at the points of total internal reflection on the glass surface. Thereby, the existence of the liquid can be detected by detecting the amount of light received by the light-receiving element 204.
In Japanese Patent Laying-Open No. Sho 60-216245 (1985), it is disclosed that the prism is set at such an angle that the light reflected from the water drops is incident upon the light receiving element only when the water drops adhere upon the glass surface, since the sensitivity for detecting the water drops is not enough with the conventional detection methods.
Further, in Japanese Patent Laying-Open No. Sho 62-163949 (1987), there is disclosed a construction in which there are provided two light sources, wherein the incident angle of the light from one of these is set to be greater than a critical angle for the total internal reflection to occur upon the detection surface thereof, while that the light from the other source is set to be less than the critical angle for the total internal reflection to occur upon detection surface thereof, thereby enabling discrimination of the presence or absence of water drops upon a glass surface.
Furthermore, in Japanese Patent Laying-Open No. Hei 8-261974 (1996), there is also disclosed a construction in which a transparent electrode of the so-called "comb" shape is positioned inside the glass plate to detect changes in electrostatic capacity, thereby actuating (controlling) the closing or opening of a window or a heater.
Moreover, in Japanese Patent Laying-Open No. Hei 6-509652 (1994)--later a PCT application--there is also disclosed a construction of a detection unit, such as a prism, which is attached upon the interior surface of the windshield through an intermediate layer which has two adhesive surfaces. This device also detects water drops on the outside surface of the windshield and controls the operation of a wiper for the windshield depending on the detected result.
With transparent substrates having conventional optical detection methods, one or more prisms are necessary for introducing the light into the glass to undergo total internal reflection, and these prisms must be closely contacted upon the glass surface therefore requiring excessive labor and time for the fitting or mounting operation. In particular, because almost all windshields for use in a car--or other wind-shielding glass plates used in other vehicles--are designed to have a curved surface, it is difficult to contact the prism upon the glass surface closely.
Moreover, for preventing unnecessary reflection at the boundary surface between the prism and the glass, the refractive indexes of the two also must be selected so as to be as close to each other as possible (i.e., refractive index matching). For this reason, a matching layer can be provided therebetween, but this increases the number of steps for manufacturing and brings about a disadvantage in the cost.
On the contrary, the method in which the water drops are detected by the variation of electric resistance or electrostatic capacity between the electrodes, such as disclosed in Japanese Patent Laying-Open No. Hei 8-261974 (1996), is inferior in durability and sensitivity to that of the above-described optical methods. In particular, for a windshield used in a car, it is important that the detected signal corresponds to the physical conditions actually seen by the driver or co-driver, and in this sense, optical method sensing is preferable.